partly fetched/updated mantaray nodes

[nugaon]

Currently the library only provides a utility function for loading all descendant Mantaray nodes of a given mantaray node.
At bigger a trie, this could be a problem because I have to load all nodes into memory meanwhile I may only want to change one node's value under a given path.

For this use-case there could be a utility function loadNodeAtPath, which only loads nodes until the given path.

The test case should cover the saving part (with Bee) after modification, where the mantaray node content addresses have to be changed from modified node up to the root mantaray node only.